The invention relates to a soot filtering unit in the exhaust discharge section of a diesel internal combustion engine which comprises at least two parallel soot filters and a regeneration device with at least one burner. In particular, the invention relates to such a unit wherein at least one control flap is used, to control the paths of the engine exhaust gas and the regenerative gas, at a point situated upstream from the soot filters.
Soot filtering units of the type to which the invention relates are known. For example, from U.S. Pat. No. 4,345,431 as well as from German Offenlegungsschrift 32 04 176. In the case of both of these prior art arrangements, for carrying out the combustion process, a supply of external air is delivered to the burner.
If burners are operated with air as an oxidation agent, and if these burners are to work with low air pressure, it is necessary that the control flaps for the control of the regeneration process be reliably tight which, however, with regard to the high temperatures prevailing there and/or because of the tendency toward fouling, causes difficulties in general, at least with long-term operation. Thus an effort is made to make leaky flaps reliable. The result is a higher blower output, which is generally not available in motor vehicles.
On the other hand, it has been shown that, with the use of an exhaust gas afterburner through which all of the exhaust gas of the internal combustion engine flows, for the regeneration process such burners must have a high burner output for the generation of hot combustible gases. At the same time, the output must still be controllable to be able to meet the desired requirements for regeneration in this application. Also, if burners are used to regenerate soot filters, the entire exhaust gas unit should be adapted to the burner features or vice versa to guarantee a reliable operation of the burner for the regeneration process of the soot filters.
Thus, a primary object of the invention is to enable the difficulties, described above, to be overcome while providing a soot filtering unit that makes possible, even with leaky control flaps, a reliable regeneration of the soot filters in a structurally simple way and without the exhaust gas unit and the burner devices having to be provided with complicated and cost-intensive measures. A further important object of the invention is to provide an arrangement wherein one or more low output burners can be used while enabling maximum operating reliability to be attained.
According to preferred embodiments of the invention, those objects are achieved by a soot filtering unit in the exhaust discharge section of a diesel internal combustion engine of the type having at least two parallel soot filters and a regeneration device with at least one burner, and at least one control flap is placed upstream from the soot filters, for controlling the regeneration process, wherein the burner is operated with exhaust gas and is placed downstream from the control flap(s).
The soot filtering arrangement according to the present invention offers the advantage that the burners are operated with exhaust gas, instead of external air, as the oxygen supplier for the burners, so that there is no great need to ensure flap tightness, and the requirement for an air delivery means, such as a blower or compressor, is avoided. Even if the control flaps in such a soot filtering unit are leaky, or become leaky over time, this poses no difficulties during operation of the burner since the burner then receives just possibly only a slightly greater amount of exhaust gas for its operation. Since the burner (seen in the direction of the exhaust gas stream) is placed behind the control flap for the regeneration process, and this flap is closed for carrying out the regeneration process, during regeneration the burner can be operated according to requirements in a controllable way without exhaust gas from the internal combustion engine constantly flowing through it. The amount of exhaust gas necessary for the operation of the burner can be fed to the burner in many ways, which will be explained in more detail below. Since the burner is intended for the regeneration of a so-called double soot filtering unit, in which two parallel soot filters are used, a desired amount of gas can be fed to the burner. This means that the burner output can be set in any way and thus an optimal regeneration behavior can be established.
Preferably, an exhaust gas feed pipe is connected to the burner that branches off from the exhaust gas line of the diesel internal combustion engine, at or upstream from the branching point to the parallel soot filters. Through this exhaust gas feed pipe, exhaust gas can be fed, optionally adjustably, to the combustion air intake of the burner for operation during the regeneration process, even if the control flap assigned to the soot filter to be regenerated is closed and the other soot filter is used for soot filtering.
According to an advantageous configuration according to the invention, a single burner preferably serves the two soot filters so that the structural volume of such a soot filtering unit can be reduced overall and thus, naturally, the costs for such a unit also can be reduced.
To guarantee that operation of the burner is largely independent of and uninfluenced by the design of the exhaust gas unit of the internal combustion engine, a flow control device may be placed in the exhaust gas feed pipe to the burner. This flow control device is, preferably, formed by a flap that regulates the amount of exhaust gas supplied.
If, as indicated above, one burner is used for the regeneration of both soot filters in an alternating sequence, then, advantageously, at least one flap is placed downstream from the burner to guide the high temperature gases produced by the burner to the one of the soot filters to be regenerated, where the temperature of these gases is used to remove the soot collected. In this case, the design can be made in such a way that, in the area of the burner outlet, a branch pipe section is provided and the high temperature gases produced by the burner are conveyed by appropriate positioning of the flap in the branch pipe section to direct these gases to the respective soot filter to be regenerated.
According to an especially advantageous configuration according to the invention, a single flap is provided that controls the path of the engine exhaust gas and that of the regeneration gas. With such a design, at the area where the exhaust pipe branches to the parallel soot filters, only a single flap is then necessary. There are no great demands ,for tightness to be placed even on such a flap, since, in case the flap becomes leaky, it only results in the high temperature regeneration gases supplied by the burner being mixed with a small portion of exhaust gas, specifically that from the leakage at the flap. Such a result hardly impairs the operation of the burner since a higher gas pressure is always present in the burner.
According to an alternative configuration, the burner outlet is connected to separate gas feed pipes for each filter. In each of these feed pipes at least one flap is placed, so that by controlling the position of these flaps the soot filter that is desired to be regenerated can be supplied with the high temperature gases from the burner outlet for starting the regeneration process.
A further embodiment according to the invention provides a design wherein a burner is placed upstream from each filter so that, seen overall, with a double filter unit, two burners are used which are operated independently of each other for soot filter regeneration. This results in the advantage that the respective burner can be placed directly in the exhaust gas pipe section leading to the filter, and supplying the burner with exhaust gas for combustion purposes occurs by a calculated adjustable leakage area of the upstream control flap. Thus, in this case, there is consciously a leakage opening present at the control flaps and the amount of combustible gas from the burner equals the amount of gas leaked from the respective control flap. With such a configuration, no additional exhaust gas feed pipes to the burners are needed and, in particular, the control flaps can consciously be made leaky.
According to a suitable further embodiment according to the invention, the exhaust gas line branches upstream from each burner and a single flap is placed in the branching point as a control common to both burners. Also, with this design the leakage current at this common flap corresponds to the gas stream required to operate the burner with exhaust gas. This results in the advantage over the previously described configurations that, for structural simplification, only a single flap is needed that acts both as a control flap for the soot filter regeneration and for supplying an amount of exhaust gas to operate the burner.
According to another alternative configuration according to the invention, a control flap is placed in each exhaust gas pipe section leading to a filter, and preferably the burners in each case do not lie directly in the exhaust gas stream, but rather the burner outlet of each burner discharges into a respective exhaust gas pipe section and the burners are supplied with exhaust gas by separate feed pipes so that the burner, during normal filter operation, has practically no exhaust gas flowing through it or around it. Consequently, interruptions in the burner operation, for example, caused by fouling, can more easily be avoided and the burner does not lead to increased flow resistance for the engine exhaust gas. By insertion of a control flap, of course, flow-through of engine exhaust gas, when the burner is in, can be almost completely avoided.
A suitable configuration according to the invention is wherein the regeneration device is placed between an exhaust gas receptacle, which preferably acts as a front muffler, and the parallel soot filters. In the configurations wherein exhaust gas is separately supplied to the burner or burners through an exhaust gas feed pipe, these exhaust gas feed pipes are connected to the receptacle. If, on the other hand, the burner is in the exhaust gas stream to the respective soot filter, the amount of exhaust gas fed to each burner may be controlled with the aid of the upstream control flaps, which are adjustable with regard to the degree that they are open for that purpose.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.